Reticulated polyurethane foam products have been used for many years as explosion suppression materials in the fuel tanks and containers of gasoline and kerosene powered vehicles. The reticulated foam is a three dimensional plastic material consisting of a plurality of strands which are interconnected at spaced apart points to define void spaces or pores. The product generally has a void volume of more than 80% and preferably more than 90%. The reticulated foam material is installed inside the fuel tank to occupy between about 50% and 100% of the interior dimensions of the tank and serves to inhibit the rapid and uncontrolled spread of a flame front when a spark is introduced into the fuel mixture. Thus, polyurethane foams and foam linings are recognized as an important safety feature in combustion technology, especially in the fuel tanks of military and racing vehicles which are often operated under incendiary or static electric discharge conditions. The reticulated urethane foam minimizes the danger of fire or explosion resulting from exposure to static electric discharges which often occur during operation or fueling, or as the result of sparks that may be generated in crashes.
Within a fuel containment area provided with a reticulated polyurethane foam, fuel is often subject to vibration and turbulent motion. The foam tends to suppress fuel agitation due to vehicular motion, but static charges can build up within the tank or containment area until they overcome air resistance, and dangerous static electricity discharges can occur, for example during a refueling operation. A static discharge, for example between an ungrounded fuel hose nozzle and the metal frame of the vehicle or tank, can damage sensitive electrical equipment, or worse, can trigger an explosion within the tank. This problem is recognized in Martel et al., Static Charge in Aircraft Fuel Tanks, Technical Report No. AFWAL-TR-80-2049 (September, 1980). Therefore, there is a definite if not urgent need for reliable and long-lasting means for safely controlling static charges in the vicinity of combustion fuels and fuel gases, especially during fueling operations.
The polyurethane foams conventionally used as fuel tank filler materials are non-conductors having high electrical resistivity, e.g., a volume resistivity of greater than 10.sup.13 ohm cm. Therefore, they cannot dissipate or control static charges. Indeed, the high resistivity of conventional foams may contribute to internal explosions caused by static build-up and discharge, even while tending to suppress or contain explosions.
Antistatic polyurethane foams which seek to achieve this purpose are known. Unfortunately, the known compositions and methods suffer from degradation and failure because they rely on antistatic agents that are not permanent; they are too easily removed from the foam structure by washing or by mechanical abrasion, or they degrade rapidly with normal aging and become ineffective as antistatic materials.
One commercially available antistatic flexible foam is produced by incorporating quaternary amines into the foam as an additive, by swelling the finished foam, as described for example in Volz, U.S. Pat. No. 4,578,406; or by using post foaming topical coatings such as conductive carbon-containing surface coatings, described as prior art in the Volz disclosure. Both of these known compositions and methods have certain drawbacks, such as poor resistance to extraction by washing and lack of resistance to mechanical abrasion. Moreover, these prior art foam compositions require a post-foaming treatment in order to impart good electrical conductivity to the foam, i.e. a relatively low electrical resistivity on the order of 10.sup.12 ohm cm or less. Additionally, some of the known antistatic foams can be very sensitive to humidity.
Fuji et al., U.S. Pat. No. 3,933,697 discloses an antistatic polyurethane foam containing a quaternary ammonium salt as the antistatic agent. Although the Fuji patent indicates that the quaternary additive can be incorporated into the foam forming reactants, it has been found that foams which depend upon quaternary salts for their electrical conductivity properties do not retain such properties when the foam is exposed to aqueous or solvent solutions for extended periods of time. Indeed, the known quaternary salts are water soluble, and wash too readily from the foam.
Berbeco, U.S. Pat. No. 4,301,040 discloses a conductive polyurethane foam incorporating finely divided conductive particles. However, it has been found that the addition of an effective antistatic amount of finely divided conductive particles results in severe deterioration of the physical properties of the foam material. Foams of sufficiently low resistivity to provide satisfactory electrical conductivity or antistatic properties (less than 10.sup.12 ohm cm) are difficult to obtain using known procedures and tend to lose their antistatic electrical properties upon exposure to high humidity or solvents.
Other conductive compounds are known to be useful in combination with solid polymers, including polyurethane resins, as opposed to polyurethane foams. For example, British Pat. No. 1,158,384; and (R. Knoesel et. al.) Bul. Soc. Chim. Fr. 1969 (1) 294-301, disclose the use of ethenetetracarboxy nitrile, also known as tetracyanoethylene (TCNE) to increase the conductivity of special donor polymer resins such as polydimethylaminostyrene and polyvinylphenothiazine.
German Offenlegungsschrift 28 38 720 discloses selectively conductive solid epoxide or polyurethane casting resins containing TCNE. This reference teaches that TCNE can effect the electrical conductivity of solid synthetic resins as electron acceptors. Solid epoxy or polyurethane resins can be combined with TCNE, and the TCNE polyurethane resin compositions are shown to have an electrical conductivity of about 0.38.times.10-.sup.10 (ohm cm).sup.-1, which corresponds to a resistivity of 2.6.times.10.sup.10 ohm cm.
These patents do not disclose or suggest that TCNE or picric acid can be combined in situ with foam-forming ingredients as charge transfer agents, to form a conductive polyurethane foam product, nor is there any suggestion that such a foam product could retain its electrical conductivity properties during the exothermic foam forming reaction (in which reaction temperatures may reach 300.degree. F. or higher for several hours) or the subsequent thermal reticulation treatment in which the solidified foam mass is exposed to momentary plasma level temperatures exceeding 2000.degree. C. and the internal temperature of the foam material may reach 400.degree. F. or more.
While the use of conductivity enhancing electron acceptor compounds (charge transfer agents) such as TCNE in solid polyurethane resins is known, the permanent and in situ incorporation of TCNE or picric acid with polyurethane foam reactants to form a permanent electrically conductive polyurethane foam having a resistivity of less than about 10.sup.12 ohm cm is not disclosed or suggested by the prior art.